Question

1).What are the different laser systems in today's’ market? A). Give two examples for each kind of laser and the usage of each laser). State the wavelength and frequency of each laser system C). What is the active medium of each laser? D). Explain the lasing process of the laser system, give enough details explain the lasering process, show the energy levels, the transition between the levels to produce the laser lines.Identify the frequency and wavelength of each level.

2.What are the four characteristics of the laser?Explain each one in details.

3.What is the difference between vibrational levels and rotational levels?

4.Compare the energy of the vibrational levels to the rotational level.

5.Do the vibrational and rotational levels exist on both gas laser and solid-state laser?Why or why not. Explain.

6.Explain the lasering process on Carbon dioxide laser, Nd: YAG laser, and HeNe laser in details and show the energy levels for lasing for example in Co2laser the 9.6 and 10.6-micron laser lines.

Answer 1

Ans1. (A) There are many types of lasers available for research, medical, industrial, and commercial uses. Lasers are often described by the kind of lasing medium they use:

• Solid state lasers have lasing material distributed in a solid matrix, e.g., the ruby or neodymium-YAG (yttrium aluminum garnet) lasers. The neodymium-YAG laser emits infrared light at 1.064 micrometers.
• Gas lasers have a primary output of a visible red light. CO₂ lasers emit energy in the far-infrared, 10.6 micrometers, and are used for cutting hard materials. eg. CO₂ Laser, He-Ne Laser
• Excimer lasers (the name is derived from the terms excited and dimers) use reactive gases such as chlorine and fluorine mixed with inert gases such as argon, krypton, or xenon. When electrically stimulated, a pseudomolecule or dimer is produced and when lased, produces light in the ultraviolet range.

• Dye lasers use complex organic dyes like rhodamine 6G in liquid solution or suspension as lasing media. They are tunable over a broad range of wavelengths.

• Semiconductor lasers, sometimes called diode lasers, are not solid-state lasers. These electronic devices are very small and use low power. They may be built into larger arrays, e.g., the writing source in some laser printers or compact disk players.

(B) Wavelengths of Laser:

• Solid state lasers: 200nm to 4 micrometers depending upon the material used.

• Gas lasers: 300 nm to 8 micrometers

• Excimer lasers: 200nm to 400 nm

• Dye lasers: 300nm to 700nm

• Semiconductor lasers: 400nm to 20 micrometers

(C) Active medium of each type of laser is alreay explained in part (A).

(D) Lasing Process of LASERs: The principle of a laser is based on three separate features: a) stimulated emission within an amplifying medium, b) population inversion of electronics and c) an optical resonator.

                                                                                           (1)

where ? is the reduced plank constant.

Conversely, a photon with a particular frequency satisfying above eq(1) would be absorbed by an electron in the ground state. The electron remains in this excited state for a period of time typically less than 10^-6 second. Then it returns to the lower state spontaneously by a photon or a phonon. These common processes of absorption and spontaneous emission cannot give rise to the amplification of light. The best that can be achieved is that for every photon absorbed, another is emitted.

                        fig1                         fig(2)

1. Spontaneous Emission and Stimulated Emission: According to the quantum mechanics, an electron within an atom or lattice can have only certain values of energy, or energy levels. There are many energy levels that an electron can occupy, but here we will only consider two. If an electron is in the excited state with the energy E₂ it may spontaneously decay to the ground state, with energy E₁, releasing the difference in energy between the two states as a photon. (see Fig1) This process is called spontaneous emission, producing fluorescent light. The phase and direction of the photon in spontaneous emission are completely random due to Uncertainty Principle. The angular frequency ? and energy of the photon is:    

Alternatively, if the excited-state atom is perturbed by the electric field of a photon with frequency ?, it may release a second photon of the same frequency, in phase with the first photon. The atom will again decay into the ground state. This process is known as stimulated emission. (see Fig.2)

The emitted photon is identical to the stimulating photon with the same frequency, polarization, and direction of propagation. And there is a fixed phase relationship between light radiated from different atoms. The photons, as a result, are totally coherent. This is the critical property that allows optical amplification to take place.

All the three processes occur simultaneously within a medium. However, in thermal equilibrium, stimulated emission does not account to a significant extent. The reason is there are far more electrons in the ground state than in the excited states. And the rates of absorption and emission is proportional the number of electrons in ground state and excited states, respectively. So absorption process dominates.

      2. Population Inversion of the Gain Medium: If the higher energy state has a greater population than the lower energy state, then the light in the system undergoes a net increase in intensity. And this is called population inversion. But this process cannot be achieved by only two states, because the electrons will eventually reach equilibrium with the de-exciting processes of spontaneous and stimulated emission.

Instead, an indirect way is adopted, with three energy levels (E₁<E₂<E₃) and energy population N₁, N₂ and N₃ respectively. (see Fig.3) Initially, the system is at thermal equilibrium, and the majority of electrons stay in the ground state. Then external energy is provided to excite them to level 3, referred as pumping. The source of pumping energy varies with different laser medium, such as electrical discharge and chemical reaction, etc.

In a medium suitable for laser operation, we require these excited atoms to quickly decay to level 2, transferring the energy to the phonons of the lattice of the host material. This wouldn’t generate a photon, and labeled as R, meaning radiationless. Then electrons on level 2 will decay by spontaneous emission to level 1, labeled as L, meaning laser. If the life time of L is much longer than that of R, the population of the E₃ will be essentially zero and a population of excited state atoms will accumulate in level 2. When level 2 hosts over half of the total electrons, a population inversion be achieved.

           fig(3)                                                                        fig(4)

Because half of the electrons must be excited, the pump system need to be very strong. This makes three-level lasers rather inefficient. Most of the present lasers are 4-level lasers, see Fig.4. The population of level 2 and 4 are 0 and electrons just accumulate in level 3. Laser transition takes place between level 3 and 2, so the population is easily inverted.

In semiconductor lasers, where there are no discrete energy levels, a pump beam with energy slightly above the band gap energy can excite electrons into a higher state in the conduction band, from where they quickly decay to states near the bottom of the conduction band. At the same time, the holes generated in the valence band move to the top of the valence band. Electrons in the conduction band can then recombine with these holes, emitting photons with an energy near the band gap energy.(see Fig.5)

                   fig(5)

    3. Optical Resonator: Although with a population inversion we have the ability to amplify a signal via stimulated emission, the overall single-pass gain is quite small, and most of the excited atoms in the population emit spontaneously and do not contribute to the overall output. Then the resonator is applied to make a positive feedback mechanism.

An optical resonator usually has two flat or concave mirrors, one on either end, that reflect lasing photons back and forth so that stimulated emission continues to build up more and more laser light. Photons produced by spontaneous decay in other directions are off axis so that they won’t be amplified to compete with stimulated emission on axis. The "back" mirror is made as close to 100% reflective as possible, while the "front" mirror typically is made only 95 - 99% reflective so that the rest of the light is transmitted by this mirror and leaks out to make up the actual laser beam outside the laser device.

More importantly, there may be many laser transitions contribute in the laser, because of the band in solids or molecule energy levels of organics. Optical resonator also has a function of wavelength selector. It just make a standing wave condition for the photons:  

                                                                                                             (2)

where L is the length of resonator, n is some integer and ? is the wavelength. Only wavelengths satisfying eq(2) will get resonated and amplified.

Ans2. Laser light has four unique characteristics that differentiate it from ordinary light: these are

1. Coherence

   We know that visible light is emitted when excited electrons (electrons in higher energy level) jumped into the lower energy level (ground state). The process of electrons moving from higher energy level to lower energy level or lower energy level to higher energy level is called electron transition.

   In ordinary light sources (lamp, sodium lamp and torch light), the electron transition occurs naturally. In other words, electron transition in ordinary light sources is random in time. The photons emitted from ordinary light sources have different energies, frequencies, wavelengths, or colors. Hence, the light waves of ordinary light sources have many wavelengths. Therefore, photons emitted by an ordinary light source are out of phase.
   In laser, the electron transition occurs artificially. In other words, in laser, electron transition occurs in specific time. All the photons emitted in laser have the same energy, frequency, or wavelength. Hence, the light waves of laser light have single wavelength or color. Therefore, the wavelengths of the laser light are in phase in space and time. In laser, a technique called stimulated emission is used to produce light.
   Thus, light generated by laser is highly coherent. Because of this coherence, a large amount of power can be concentrated in a narrow space.

2. Directionality

   In conventional light sources (lamp, sodium lamp and torchlight), photons will travel in random direction. Therefore, these light sources emit light in all directions.

   On the other hand, in laser, all photons will travel in same direction. Therefore, laser emits light only in one direction. This is called directionality of laser light. The width of a laser beam is extremely narrow. Hence, a laser beam can travel to long distances without spreading.
   If an ordinary light travels a distance of 2 km, it spreads to about 2 km in diameter. On the other hand, if a laser light travels a distance of 2 km, it spreads to a diameter less than 2 cm.

3. Monochromatic

   Monochromatic light means a light containing a single color or wavelength. The photons emitted from ordinary light sources have different energies, frequencies, wavelengths, or colors. Hence, the light waves of ordinary light sources have many wavelengths or colors. Therefore, ordinary light is a mixture of waves having different frequencies or wavelengths.

   On the other hand, in laser, all the emitted photons have the same energy, frequency, or wavelength. Hence, the light waves of laser have single wavelength or color. Therefore, laser light covers a very narrow range of frequencies or wavelengths.\

4. High Intensity

   You know that the intensity of a wave is the energy per unit time flowing through a unit normal area. In an ordinary light source, the light spreads out uniformly in all directions.

   If you look at a 100 Watt lamp filament from a distance of 30 cm, the power entering your eye is less than 1/1000 of a watt.

   In laser, the light spreads in small region of space and in a small wavelength range. Hence, laser light has greater intensity when compared to the ordinary light.

   If you look directly along the beam from a laser (caution: don’t do it), then all the power in the laser would enter your eye. Thus, even a 1 Watt laser would appear many thousand times more intense than 100 Watt ordinary lamp.